The present disclosure relates generally to systems and methods for aligning medical devices with target tissue. In particular, the instant disclosure relates to systems and methods for aligning medical devices, such as directional ablation catheters, to target tissue by monitoring irrigant backpressure. Directional medical devices require positioning the active end, e.g. ablation or sensing end, toward the tissue. In the context of ablation catheters, for example, such monitoring of irrigant backpressure can be utilized to more effectively direct therapy towards a target tissue and/or to ensure that the device is properly aligned to adequately evaluate the efficacy of the therapy via an imaging or sensing element, such as an electrode or transducer.
Catheters are common medical tools that have been used for many years. Such devices are employed in medical procedures, for example, to examine, diagnose, and treat while positioned at a specific location within the body that would otherwise be inaccessible without more invasive procedures. In such procedures, a catheter is typically first inserted into a vessel near the surface of the body and then guided to a specific location within the body. Once positioned at the desired location, the catheter may be used, for example, to convey an electrical stimulus to a selected location within the human body and/or to monitor various forms of electrical activity within the body.
Using catheters in medical procedures, such as cardiac ablation, for the treatment of certain types of cardiac arrhythmia has become increasingly more common. Catheter ablation is based on the idea that by ablating (i.e., destroying) abnormal tissue areas in the heart, the heart's electrical system can be altered to return the heart to normal rhythm. During catheter ablation therapy, the catheter is typically inserted into an artery or vein in the leg, neck, or arm of the patient and then threaded, sometimes with the aid of a guide wire or introducer, through the vessels until a distal tip of the catheter reaches the desired location for the medical procedure in the heart.
During conventional catheter ablation procedures, an energy source is in contact with cardiac tissue to heat the tissue and create a permanent scar or lesion that is electrically inactive or non-contractile. These lesions are designed to interrupt existing conduction pathways commonly associated with arrhythmias within the heart. The particular area for ablation depends on the type of underlying arrhythmia. The use of RF ablation electrode catheters for ablating specific locations within the heart has been disclosed in, e.g., U.S. Pat. Nos. 4,641,649, 5,228,442, 5,231,995, 5,263,493, and 5,281,217.
Many conventional ablation procedures use a single electrode secured to the tip of an ablation catheter. It has become increasingly more common to use multiple electrodes affixed to the catheter body, however, with such ablation catheters often containing a distal tip electrode and a plurality of ring electrodes as disclosed in, e.g., U.S. Pat. Nos. 4,892,102, 5,228,442, 5,327,905, 5,354,297, 5,487,385, and 5,582,609.
Many conventional ablation catheter tips may be placed upon the tissue and activated to deliver ablative action regardless of the catheter orientation with respect to rotation about the catheter tip axis. That is, because many of these devices are omnidirectional in their function, the orientation of the catheter tip relative to the target tissue is not a significant factor in their efficacy. For example, some conventional RF tips include an RF electrode that is symmetric about its longitudinal axis in order to ablate uniformly without regard to tip rotation. In such cases, the rotational alignment of the catheter tip does not have a significant impact, and thus there is often no rotational alignment requirement for the catheter.
A variety of energy sources can be used to supply the energy necessary to ablate cardiac tissue and create a permanent lesion. Such energy sources include direct current, laser, microwave, and high intensity ultrasound. Because of problems associated with the use of DC current, radiofrequency (RF) alternating current has become a common source of energy for many ablation procedures. The use of RF energy for ablation has been disclosed, e.g., in U.S. Pat. Nos. 4,945,912, 5,242,441, 5,246,438, 5,281,213, 5,281,218, and 5,293,868.
In contrast to catheters that are rotationally symmetric about their longitudinal axis, some emerging catheter designs deliver unidirectional therapy or sensing and thus must be rotationally aligned with the target tissue to be ablated and are configured to deliver therapy or sensing in one or more specific aligned directions. In some emerging procedures, for example, such catheters are used for delivering at least one of a diagnostic or therapeutic function in a directional manner because the inherent operation of the catheter tip is directional rather than omnidirectional. An example of a directional therapeutic function is a directed laser beam that ablates in one direction. For example, the directed laser beam may be used for directing ablation therapy from a side or away from an end of the tip towards a target tissue site. An example of a directional diagnostic or sensing function is a diagnostic ultrasound pinger directed towards a target tissue location for pinger ultrasonic-echo provided feedback on the formation of an ablation lesion or other tissue characteristics.
In some cases, the catheter tip may have both ablative as well as diagnostic functionality, one or both of which may be directional in nature, requiring rotating or angulating the tip to aim towards a particular position on the tissue. This aiming of the catheter can be referred to as directional rotational or angular alignment. It should be understood that once a catheter is placed against a target tissue surface it develops a contact force regardless of its rotational or angular orientation or need thereof. Alignment is about aiming or directing whereas contact-force is about contact force or pressure regardless of the tip's alignment.
In the context of ultrasonic pingers, for example, it is often necessary to have good alignment such that the pinging transducer beam is directed towards and squarely into the tissue and the pinger contacts that tissue consistently with at least a few grams of force. If the tip containing the pinger is also an RF ablating tip then a good ablator tissue contact force of 20-30 grams minimum is needed such that the ablation current passes into the tissue with a reasonably low electrical contact impedance. Although the terms “contact force” and “contact pressure” are used interchangeably herein, it should be understood that the force divided by the actual tissue contact area yields the contact pressure. In some RF ablation procedures, for example, a tip contact force of about 20 grams minimum may be required for reproducible RF contact to tissue.
Determination of contact force between the catheter ablator tip and the target tissue has long been indirectly done by (a) monitoring the electrical contact impedance at the RF ablation frequency, and in some cases also (b) monitoring the apparent deformed shape of the catheter, such as in an X-ray fluoroscopy image. More recently, a number of direct optical methods utilizing optical fibers have been suggested such as that of Endosense Inc. in which a number of interferometer optical fibers are used to monitor tip-spring displacements (tip bending and axial deflections), which can be used to derive tip forces. Approaches which utilize three or more such optical interferometer fibers plus dedicated LEDs and photodiodes can become expensive to manufacture, are fragile, and do not leave much room for other important catheter components, such as catheter steering wires and fluid lumens.
Other means for determining tissue contact and catheter alignment are pinging acoustic transducers mounted in or adjacent the catheter tip to acoustically detect lesion volume and tissue-contact, as well as indirectly tissue contact force. The acoustic pulse-echo approach may also permit a clinician to discern the lesion state at specific tissue depths when time-delay pulse-echo range data is available. This also allows for direct measurement of tissue thickness or organ proximity